During the past decade, the network capacity of the internet has grown faster than the traffic itself, but it has not always grown in the areas of greatest need. For instance, it has been easier to add capacity to the “long-haul backbone” that connects major Internet hubs in different cities than to add capacity to networks within cities themselves, i.e., within “metro-area networks” or the “last mile” to people's homes and offices. Optical communication is currently mostly employed in the core networks between the major metropolitan areas because deployment within the metropolitan areas has been cost-prohibitive. This has led to the current situation in which there is too much capacity in the long-haul pipes and unmet demand in cities and suburbs where the people who could use the bandwidth actually live and work. The profit-starved large carriers are in desperate need of new equipment that can lower their operating costs and provide new services like bandwidth on demand. (See, e.g., Schonfeld, “Can the Telecoms Bounce Back?”, Business 2.0, 52-53 (December 2001)). Optical polymers have become a major player in the race to produce optical waveguides, switches, modulators, and the like, because they can provide a major cost and performance advantage over the competing inorganic technologies in the field. Such newer and cheaper technology potentially will extend the reach of broadband.
To fabricate such polymer-based waveguide devices, two main types of polymeric materials are needed: (1) cladding layers, and (2) active layers that are sandwiched between two cladding layers. Waveguides need to have a cross-section dimension of 8 microns, similar to that of a fiber optic core. At the same time waveguides need to be single-mode. This means that the refractive index of the core need only be slightly higher than the cladding layers (Δn≈0.005). (See, e.g., PCT International Application No. 01/06305). The index of refraction of the active polymer material used by various users and developers ranges between 1.5 to 1.7. Cladding layers are needed that have the same range of refractive index yet dissolve in totally non-interacting solvent systems to allow sequential layering without affecting underlying layers. Cladding materials which are processed in water would be ideal for this purpose.
Polyimides are a class of high performance materials that have exceptional properties. For instance, polyimides possess high optical transparency, low dielectric constants, high mechanical strength, oxidative and hydrolytic stability, as well as thermal stability. Polyimides have been used as electrical insulators, coatings, adhesives and in many other applications in the semiconductor industry. (See generally, Mittal (Ed.), “Polyimides”, Plenum Press, New York, Vols. 1, 2 (1984)). Typically, polyimides have been synthesized by polycondensation of dianhydrides and primary diamines via a poly(amic acid) precursor which subsequently can be converted into a polyimide either thermally or chemically (Bower et al., J. Poly. Sci. A., 1, 3135 (1963); Sroog et al., J. Polym. Sci. A., 3, 1373 (1965)). However, it is well known that aromatic polyimides are insoluble in conventional solvents and infusible up to their decomposition temperature. Therefore, much effort has been devoted to synthesize organo-soluble, aromatic polyimides. The enhanced solubility of polyimides in organic solvents is achieved by a carefully modifying both the dianhydride and the diamine structures. These structural modifications can significantly reduce the electronic conjugation and loosen the intermolecular packing, while retaining the relative rigidity and linearity of the chain backbone configuration (Cheng et al., TRIP, 5, 51 (1997)).
However, cladding layers using organo-soluble polyimide currently have two problems. First, they generally suffer from low adhesion between adjacent layers. Second, they can interfere with (ie., partially dissolve) the active layer, especially if both layers are soluble in polar aprotic solvents such as N-methylpyrrolidone (NMP), dimethylformamide (DMF), and the like.
Accordingly, there exists a need for water-soluble polyimides, which would solve this problem of decreased adhesion and layer incompatibility upon processing. The development of such polymers would assist in making an all-polymeric optical integrated circuit and processor possible, as well as additional optical components for the telecommunications industry, including wavelength division multiplexers, polymer-waveguide biosensors, and the like. The polymers also optionally could be employed for other applications. For instance, the polymers could be of use in the electronics industry to cover and protect integrated circuits, and obviate and issues regarding solvents. The polymers also could be of use, e.g., as anti-reflective coatings for flat screen displays.
The present invention thus is directed, amongst other things, to water-soluble polymers, methods of obtaining such polymers, and methods of using such polymers, e.g., in optical devices. In particular, the present invention provides polyimides bearing functional groups which preferably can be converted into water-soluble salts, particularly ammonium salts, by reaction of various polyimide carboxylic acids with tertiary amines or ammonium hydroxide. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein. The description and examples are provided to enhance the understanding of the invention, but are not intended to in any way to limit the scope of the invention.